This invention relates to a process and a device for producing an optical element having a photonic-band structure, particularly an optical element comprising a three-dimensional photonic crystal having a desired crystal structure easily and in a short period of time, and it also relates to an optical element produced by using the process and the device.
Furthermore, the present invention relates to an optical element and an optical demultiplexer. More specifically, it relates to an active optical element and an optical demultiplexer that have achieved an optical switching function by changing a photonic-band structure by switching external fields such as light and an electric field in a photonic crystal.
In the structure called xe2x80x9cphotonic crystalxe2x80x9d in which two types of optical media having different refractive indices are arranged periodically at a wavelength order of light, the relationship between the wave number of light and its frequency, i.e. photon energy, shows a band structure due to periodic changes in the refractive indices. This phenomenon is similar to the phenomenon that electron energy in a semiconductor shows a band structure in a periodic potential.
The photonic crystal is significantly characterized by its optical properties since it is capable of making the so-called xe2x80x9cphotonic-bandgapxe2x80x9d in which light does not transmit in any directions (E. Yablonovitch, Phys. Rev. Lett. 58(20), 2059(1987)) appear and has very high degrees of optical anisotropy and dispersibility. Thus, by taking advantage of such properties, there have been proposed the control of natural light and an optical waveguide, a polarizer and an optical demultiplexer that have a very small radius of curvature at the corner, and expectations are being raised about their applications to a variety of fields.
Heretofore, however, there has not been available an effective process for producing a photonic crystal, particularly a three-dimensional photonic crystal, in which the refractive indices have a periodic structure at the wavelength order of light, in the form of a crystal structure suitable for the application of an optical element. This has been a factor that hinders the commercialization of the photonic crystal and an optical element using the same.
To improve the above situation, there have recently been made several reports on the production of a photonic crystal at the wavelength order of light. Representative among them are on the following three processes.
(1) A process for producing a photonic crystal by removing a solvent from a colloidal solution containing silicon oxide fine particles to crystallize the silicon oxide fine particles. This process takes advantage of the self-arrangement of silicon oxide fine particles, and the photonic crystal produced is called xe2x80x9copal typexe2x80x9d. By this process, a crystal having a high repetition number can be produced relatively easily (H. Miguez et al., appl. Phys.
Lett. 71(9), 1148(1997)). However, in this process, the silicon oxide fine particles are not arranged with high reproducibility and high reliability, and a crystal structure cannot be selected freely.
(2) Wood-Pile process (S. Noda et al. Jpn. J. Appl. Phys., 35, L909(1996)). In this process, by using a semiconductor micromachining technique, a structure comprising a plurality of arranged square timbers is formed on each of two substrates, the substrates are bonded to each other in such a manner that the square timbers on one substrate are faced at right angles with the square timbers on the other substrate, and one of the substrates is removed by etching to form a structure comprising two layers of xe2x80x9csquare timbersxe2x80x9d. Similarly, a substrate having xe2x80x9csquare timbersxe2x80x9d arranged on the surface is prepared, and a layer of square timbers is piled up by repeating bonding with accurate positioning and etching. It has been found that a diamond structure which opens a photonic-bandgap in all directions can be formed by this process. This process, however, requires a micromachining process which is complicated and time-consuming, and there is a limit for the number of repeating periods that can be actually formed.
(3) A process called xe2x80x9cautocloningxe2x80x9d process (Kawakami et al., Japanese Patent Application Laid-Open No. 335758/1998). In this process, a two-dimensional, periodic convexo-concave pattern is formed on a substrate made of quartz or a semiconductor by lithography, and a number of thin films are laminated thereon while the underlying convexo-concave pattern is reproduced by bias sputtering. Thus, a three-dimensional periodic structure is formed both in the surface direction of the substrate on which the convexo-concave pattern has been engraved at the beginning and in the laminating direction perpendicular to the surface. This process is more reliable and more excellent in terms of reliability and reproducibility than the process for producing the opal-type photonic crystal, and does not require a micromachining process which is as complicated and time-consuming as that in the Wood-Pile process. Therefore, this process is capable of producing a photonic crystal which has a relatively large number of periods in the laminating direction. However, since it is inevitable in this process that concave portions come over the concave portions of the pattern of the underlying layer and convex portions come over the convex portions of the pattern of the underlying layer, this process can realize only specific types of crystal structures and therefore cannot attain arbitrary types of crystal structures. In fact, a photonic crystal having a perfect bandgap which opens in all directions cannot be formed by this process.
Other than the above three processes, there has been proposed a process for producing a photonic crystal by taking advantage of an interference pattern of light (Tsunetomo, Koyama, Japanese Patent Application Laid-Open No. 68807/1998). In this process, a laser beam is directed onto a number of thin films laminated one-dimensionally so as to bake the interference pattern on the films, and periodic incisions are made in a perpendicular direction on the surface of the multi-layer film by taking advantage of the fusion, evaporation and ablation occurring on portions where light intensity is high to form a photonic crystal. This process is considered to be an efficient process because it can form a number of periods at a time when a periodic structure is formed by using the interference pattern of a laser. However, even this process is limited in the types of crystal structures it can form.
As described above, the conventional process taking advantage of the self-arrangement of silicon oxide fine particles has problems associated with reliability and reproducibility.
Meanwhile, since other processes require that each layer be laminated with high accuracy to form the periods of a photonic crystal, even if they succeed in the formation of the photonic crystal, it takes long time, the number of repeating periods is limited, and a desired crystal structure cannot be formed freely.
Meanwhile, the application of such a photonic crystal has also been limited heretofore.
That is, except for the three examples that will be given below, the photonic crystal has been conventionally used as a xe2x80x9cpassive elementxe2x80x9d, and they have been rarely proposed to be used as an xe2x80x9cactive elementxe2x80x9d. In other words, most of the conventionally proposed photonic crystals are determined their optical properties by the refractive-index distribution fixed in space. Therefore, in an optical demultiplexer, for example, the wavelength (frequency) of light to be transmitted in a specific direction is fixed, and the frequency of light to be derived in a specific direction has not been able to be switched. It has also not been possible to dynamically switch the direction of light from one direction of a branch placed in a waveguide to the other direction thereof.
The following three proposals use a photonic crystal as an xe2x80x9cactive elementxe2x80x9d having a switching function.
(4) One of the proposals uses a photonic crystal in which an ultrasonic generator or a thermoregulator for disturbing its periodicity and braking its band structure has been installed. It is intended by the installation of such devices to make appear or disappear the delaying effect of a photonic crystal to be used as a delay unit for light (Todori et al., Japanese Patent Application Laid-open No. 83005/1998).
(5) Another proposal uses a one-dimensional photonic crystal that has an electrooptic material sandwiched between diffraction gratings having a metal film formed on the surface facing the other grating. By applying a voltage between the metal films, the refractive index of the electrooptic material changes and the position of the bandgap in a one-dimensional direction changes, whereby the transmission of light having a wavelength near the end of the band can be made ON/OFF (Todori et al., Japanese Patent Application Laid-Open No. 83005/1998).
(6) In the third proposal, a photonic crystal containing a semiconductor as its constituent is irradiated with circularly-polarized light as controlling light to change the distribution of spins in the photonic crystal material, i.e., complex refractive index thereof, whereby the photonic-band structure changes, with the result that switching of light transmitting the photonic crystal is achieved (Takeuchi, Nishikawa, Japanese Patent Application Laid-Open No. 90634/1998).
However, the above three proposals still have problems to be solved with regard to the following points.
That is, the above proposal (4) merely switches between the appearance and the disappearance of the function as the photonic crystal, and does not change actively the manner in which the function as the photonic crystal appears. Therefore, it cannot be used for controlling the direction in an optical demultiplexer or the branching in an optical waveguide.
The above proposal (5), due to its structure, can only apply to a one-dimensional photonic crystal, and cannot apply to a two-dimensional or three-dimensional photonic crystal having high dispersibility and excellent properties as waveguides.
The above proposal (6) changes the band structure by changing the complex refractive indices of the optical media constituting the photonic crystal, and cannot change the periodicity and symmetry of the photonic crystal. Therefore, it cannot induce a change in a large band structure.
As described above, the conventional photonic crystals, even when imparted with an active function, have been limited to the action of selecting whether the function itself as the photonic crystal should appear or disappear, in the number of dimensions of the photonic crystal capable of switching and to a particular controllable range. In either case, the prior arts use a process of changing only the refractive indices of optical materials of different types without changing distributions thereof, cannot switch the crystal structure and periodicity of the photonic crystal, and cannot change the structure of a photonic-band freely and dynamically.
Meanwhile, it is known that in the case where a bandgap occurs in the photonic crystal, when the spots of a photonic crystal in which periodicity is irregular are continued one-dimensionally, light is trapped within only these spots, thereby forming a fine optical waveguide that can stand sharp bending which has not been conventionally achieved (Attila Mekis et al., Phys. Rev. Lett. 77, 3787 (1996)). If a branch can be placed in such a fine optical waveguide to switch the direction of light according to its wavelength, the waveguide itself functions as an optical demultiplexer, whereby an optically functional element that is extremely useful in the integration in optical communications and optical circuits and in the simplification of production process thereof can be formed.
However, in the above prior arts, for any wavelength of light in the waveguide, the same spots in the photonic crystal always exhibit irregular periodicity, that is, they function as a waveguide. Thus, it has not been possible to use the above-described fine waveguide itself in the photonic crystal as an optical demultiplexer that functions according to wavelength.
As specifically described above, conventionally, the locations of those spots having different refractive indices, that is, the patterns of spatial changes in refractive indices, have been fixed in space and, therefore, there has been a limit to the range of change in the band structure. As a result, it has not been possible to change the band structure freely, significantly and dynamically for the active use of the photonic crystal.
That is, a technique to attain an optical element using an active photonic crystal has not been known heretofore. Further, a technique to use a fine waveguide itself in a photonic crystal as an optical demultiplexer has not been known heretofore, either.
It is the first object of the present invention to provide a novel process and a novel production device that can produce a three-dimensional photonic crystal having a period of a wavelength order of light in the form of any crystal structure with ease and a short period of time without going through the step of laminating layers of xe2x80x9ccrystalxe2x80x9d with accuracy as in the conventional process; and an optical element produced by the process and the device.
It is the second object of the present invention to provide a novel optical element that can control the band structure of a photonic crystal freely, significantly and dynamically, particularly an optical element that can control the band structure of a photonic crystal by changing the distribution pattern of complex refractive index or periodicity itself; and a novel optical demultiplexer using the waveguides in the photonic crystal.
First of all, the production process of the optical element of the present invention is a process for producing an optical element comprising a photonic crystal in which spots having different refractive indices are arranged periodically, which comprises the step of placing an optical medium, whose refractive index changes by irradiation of light or by conducting predetermined treatment after the irradiation of light according to the intensity of the applied light, in a field where light intensity changes at a period of the wavelength order of light in space and keeping the medium therein for a given time; and the step of repeating at least once the step of changing the position of the optical medium and having the above field acted on the medium again.
As the optical medium used in the present invention, there can be used one whose refractive index changes, according to the intensity of applied light, by setting the medium aside for a given time after irradiation of light, or by subjecting the medium to heat treatment, irradiation of electromagnetic wave or corpuscular radiation, or treatment with chemicals after the irradiation of light.
In the process of the present invention, the optical field where light intensity changes at a period of the wavelength order of light in space is created, for example, by interference of a laser beam. To shift the position of the optical medium for a minute distance of the wavelength order of light, a piezo element-incorporated stage that can shift the optical medium in three directions of x, y and z is used, for example.
The production device of the optical element of the present invention comprises an optical system that creates an optical field where light intensity changes in space at a period of the wavelength order of light, and a movable stage that keeps an optical medium whose refractive index changes according to the intensity of applied light in the optical field where light intensity changes periodically and that can shift the optical medium for a minute distance of the wavelength order of light in the field.
The production device of the optical element of the present invention may further comprise a light source and a detector for evaluating the optical element produced.
The optical element of the present invention that is produced by the above process and the above device is an optical element comprising a photonic crystal in which spots having different refractive indices are arranged periodically, wherein a spot with a certain refractive index which constitutes the photonic crystal is located at a lattice point of a desired crystal structure; the refractive index distribution of an optical medium located at each lattice point has a shape having projections or bulges in the directions of three different axes; the crystal structure is not a simple lattice or the shape of the refractive index distribution of an optical medium located at each lattice point is not isotropic; and the refractive index distributions of optical media located at the lattice points together forming a simple lattice have the same shape and direction.
Meanwhile, the optical element of the present invention has a structure comprising the first optical medium in which the second optical medium and the third optical medium are periodically arranged at an interval of the wavelength order of incident light. In the optical element, the relative relationship among the refractive indices of the first to third, that is, the first, the second and the third optical media is changed by changing the external field condition applied to the above structure, whereby the periodicity of the spacial distribution of the refractive indices formed in the above structure can be changed.
The phrase xe2x80x9cwavelength order of lightxe2x80x9d as used herein indicates an interval of about the same order as the wavelength of light. The interval is not significantly different from the wavelength of light, as exemplified by at least several tens of times or at most several tenths of the wavelength.
To describe the above constitution more specifically, the optical element of the present invention has a one-dimensional, two-dimensional or three-dimensional structure in which at least three types of optical media are arranged periodically. Optical materials, temperature and external field conditions are selected so that the refractive indices of at least two optical media should be different from each other to the frequency of incident light. By the application of electric field, magnetic field or pressure or the irradiation of light to the structure, or by a change in the electric field, magnetic field or pressure applied to the structure, a change in the intensity or wavelength of applied light or a change in the temperature of the structure, a combination of optical media having the largest refractive index difference change in the frequency of light inputted in the structure, or new spots having different refractive indices from the existing spots appear periodically, that is, a new periodic structure is formed after the change, or the relative ratio of the periodic peaks of the refractive indices of the media which occur in the structure changes, so that a new band structure appears in the wavelength range associated with incident light.
As for the usage form of the optical element, in the structure of the optical element, since the refractive index of the first optical medium and that of the third optical medium are substantially the same and the refractive index of the first optical medium and that of the second optical medium are substantially different to light having a given wavelength under the first external field condition, the light having a given wavelength is modulated by the periodic arrangement of the second optical medium, whereas since the refractive index of the first optical medium and that of the second optical medium are substantially the same and the refractive index of the first optical medium and that of the third optical medium are substantially different to the light having a given wavelength under the second external field condition different from the first external field condition, the light having a given wavelength is modulated by the periodic arrangement of the third optical medium.
To describe it more specifically, when the structure having a periodic structure is constituted by three types of optical media, which are each defined as the first optical medium, the second optical medium and the third optical medium and whose refractive indices are each defined as the first refractive index, the second refractive index and the third refractive index, the distribution of each of the first, the second and the third optical media in the structure has a periodic structure. In the above structure, since the refractive index of the first optical medium and that of the third optical medium are about the same and the refractive index of the first optical medium and that of the second optical medium are different in the wavelength of light inputted to the optical element, that is, the difference between the first refractive index and the second refractive index is larger than the difference between the first refractive index and the third refractive index, the periodic structure of the refractive index by which incident light is modulated in the structure is determined mainly by the periodic distribution of the second medium. Further, by the application of electric field, magnetic field or pressure or the irradiation of light to the structure, or by a change in the electric field, magnetic field or pressure applied to the structure, a change in the intensity or wavelength of applied light or a change in the temperature of the structure, the refractive index of the first medium and that of the second medium become about the same and the refractive index of the first medium and that of the third medium become different in the above wavelength, that is, the difference between the first refractive index and the third refractive index becomes larger than the difference between the first refractive index and the second refractive index, whereby the periodic structure of the refractive index by which incident light is modulated in the structure is determined mainly by the periodic distribution of the third medium instead, so that a new band structure appears in the wavelength range associated with incident light.
Further, the third optical element of the present invention has a structure comprising the first optical medium, the second optical medium arranged periodically in the first optical medium, and the third optical medium which has been substituted for and arranged in continuous spots of the periodic structure that should be formed by the second optical medium in the first optical medium. In this third optical element, since the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are substantially different to light having a given wavelength under the first external field condition, the continuous spots substituted by the third optical medium function as a waveguide to the light having a given wavelength, whereas since the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are substantially the same to the light having a given wavelength under the second external field condition different from the first external field condition, the continuous spots substituted by the third optical medium do not function as a waveguide to the light having a given wavelength.
To describe it more specifically, the above third optical element has a two-dimensional or three-dimensional structure which comprises at least two types of optical media having different complex refractive indices and in which spots formed of the same type of optical medium are arranged periodically. The irregularities in the periodic structure of this structure exist as one-dimensionally continuous spots in the structure, and the one-dimensionally continuous spots function as an optical waveguide. When at least three types of optical media are used, three optical media out of these optical media are each defined as the first optical medium, the second optical medium and the third optical medium, and the complex refractive indices of the first, the second and the third optical media in the vicinity of the frequency xcexd of light to be inputted in this waveguide are each defined as the first complex refractive index, the second complex refractive index and the third complex refractive index, this structure has a two-dimensional or three-dimensional periodic structure formed of the second optical medium in the first optical medium, a portion of the two-dimensional or three-dimensional periodic structure formed of the second optical medium is substituted by the one-dimensionally continuous spots formed of the third medium, the first complex refractive index and the second complex refractive index are different and the second complex refractive index and the third complex refractive index are also different in the vicinity of the frequency of light inputted in this optical element, and the spots substituted by the third medium function as an optical waveguide. By the application of electric field, magnetic field or pressure or the irradiation of light to the structure, or by a change in the electric field, magnetic field or pressure applied to the structure, a change in the intensity or wavelength of applied light or a change in the temperature of the structure, the second complex refractive index and the third complex refractive index become about the same while the first complex refractive index and the second complex refractive index remain different in the vicinity of the frequency xcexd, whereby the spots substituted by the third medium do not function as the irregularities in the periodic structure to incident light and therefore the spots functioning as an optical waveguide disappear. Thus, this optical element is imparted with a waveguide having a switching function.
Further, the fourth optical element of the present invention has a structure comprising the first optical medium, the second optical medium arranged periodically in the first optical medium, the third optical medium which has been substituted for and arranged in the first continuous portion of the periodic structure that should be formed by the second optical medium in the first optical medium, the fourth optical medium which has been substituted for and arranged in the second continuous portion of the periodic structure that should be formed by the second optical medium in the first optical medium, and the third continuous portion of the periodic structure which should be formed by the second optical medium and in which the periodicity of the second medium is irregular; and the first portion and the second portion are connected to the third portion. In the fourth optical element, since, under the first external field condition, the complex refractive index of the first optical medium, the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are different from one another to light having a given wavelength, and the complex refractive index of the second optical medium and the complex refractive index of the fourth optical medium are substantially the same to the light having a given wavelength, the first portion and the third portion function as waveguides to the light having a given wavelength. On the other hand, since, under the second external field condition different from the first external field condition, the complex refractive index of the first optical medium, the complex refractive index of the second optical medium and the complex refractive index of the fourth optical medium are different from one another to the light having a given wavelength, and the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are substantially the same to the light having a given wavelength, the second portion and the third portion function as waveguides to the light having a given wavelength. Thus, the fourth optical element of the present invention is capable of switching the heading direction of the light having a given wavelength which has been inputted in the third portion either to the first portion or to the second portion.
To describe it more specifically, the above fourth optical element has a two-dimensional or three-dimensional structure which comprises at least two types of optical media having different complex refractive indices and in which spots formed of the same type of optical medium are arranged periodically. The irregularities in the periodic structure of this structure exist as one-dimensionally continuous spots in the structure, and the one-dimensionally continuous spots function as an optical waveguide. When at least four types of optical media are used, these optical media are each defined as the first optical medium, the second optical medium, the third optical medium and the fourth optical medium, and the complex refractive indices of the first to the fourth optical media in the vicinity of the frequency of light to be inputted in this optical waveguide are each defined as the first complex refractive index, the second complex refractive index, the third complex refractive index and the fourth complex refractive index, this structure has a two-dimensional or three-dimensional periodic structure formed of the second optical medium in the first optical medium, a portion of the two-dimensional or three-dimensional periodic structure formed of the second optical medium is substituted by the one-dimensionally continuous portion formed of the third medium to form the first portion, another portion thereof is substituted by the one-dimensionally continuous portion formed of the fourth medium to form the second portion, one-dimensionally continuous irregularities in the periodic structure of the second medium are formed in still another portion thereof to form the third portion, and the first portion and the second portion are connected to the third portion. Since the first complex refractive index and the second complex refractive index are different, the second complex refractive index and the third complex refractive index are also different, and the second complex refractive index and the fourth complex refractive index are about the same in the vicinity of the frequency of light to be inputted in this optical element, the third portion and the first portion function as optical waveguides for the incident light. By the application of electric field, magnetic field or pressure or the irradiation of light to the structure, or by a change in the electric field, magnetic field or pressure applied to the structure, a change in the state of applied light or a change in the temperature of the structure, the second complex refractive index and the fourth complex refractive index become different and the second complex refractive index and the third complex refractive index become about the same while the first complex refractive index and the second complex refractive index remain different in the vicinity of the frequency xcexd, whereby the first portion no longer functions as the irregularity in the periodic structure to the incident light and therefore no longer functions as an optical waveguide while the second portion starts to function as an optical waveguide instead. Thus, the above fourth optical element is capable of switching the heading direction of the light inputted in the third portion between the first portion and the second portion.
Meanwhile, the optical demultiplexer of the present invention has a structure comprising the first optical medium, the second optical medium arranged periodically in the first optical medium, the third optical medium which has been substituted for and arranged in the first continuous portion of the periodic structure that should be formed by the second optical medium in the first optical medium, the fourth optical medium which has been substituted for and arranged in the second continuous portion of the periodic structure that should be formed by the second optical medium in the first optical medium, and the third continuous portion of the periodic structure which should be formed by the second optical medium and in which the periodicity of the second medium is irregular; and the first portion and the second portion are connected to the third portion. In this optical demultiplexer, since the complex refractive index of the first optical medium, the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are different from one another to light having the first wavelength, and the complex refractive index of the second optical medium and the complex refractive index of the fourth optical medium are substantially the same to the light having the first wavelength, the first portion and the third portion function as waveguides to the light having the first wavelength. On the other hand, since the complex refractive index of the first optical medium, the complex refractive index of the second optical medium and the complex refractive index of the fourth optical medium are substantially different from one another to light having the second wavelength different from the first wavelength, and the complex refractive index of the second optical medium and the complex refractive index of the third optical medium are substantially the same to the light having the second wavelength, the second portion and the third portion function as waveguides to the light having the second wavelength. Thus, this optical demultiplexer is capable of directing the light having the first wavelength or the second wavelength which has been inputted in the third portion either to the first portion or to the second portion depending on wavelength thereof.
To describe it more specifically, the optical demultiplexer of the present invention has a two-dimensional or three-dimensional structure which comprises at least two types of optical media having different complex refractive indices and in which spots formed of the same type of optical medium are arranged periodically. The irregularities in the periodic structure of this structure exist as one-dimensionally continuous spots in the structure, and the one-dimensionally continuous spots function as a waveguide. When at least four types of optical media are used in this structure and each defined as the first optical medium, the second optical medium, the third optical medium and the fourth optical medium, the frequencies of two lights to be inputted in this optical waveguide are each defined as the first frequency and the second frequency, the complex refractive indices of the first to the fourth optical media in the vicinity of the first frequency are defined as the first complex refractive index, the second complex refractive index, the third complex refractive index and the fourth complex refractive index, and the complex refractive indices of the first to the fourth optical media in the vicinity of the second frequency are defined as the fifth complex refractive index, the sixth complex refractive index, the seventh complex refractive index and the eighth complex refractive index, this structure has a two-dimensional or three-dimensional periodic structure formed of the second optical medium in the first optical medium, a portion of the two-dimensional or three-dimensional periodic structure formed of the second optical medium is substituted by the one-dimensionally continuous portion formed of the third medium to form the first portion, another portion thereof is substituted by the one-dimensionally continuous portion formed of the fourth medium to form the second portion, one-dimensionally continuous irregularities in the periodic structure of the second medium are formed in still another portion thereof to form the third portion, and the first portion and the second portion are connected to the third portion. In this case, since the first complex refractive index and the second complex refractive index are different, the second complex refractive index and the third complex refractive index are also different, and the second complex refractive index and the fourth complex refractive index are about the same in the vicinity of the first frequency of light to be inputted in this optical element, the third portion and the first portion function as optical waveguides for the incident light, whereas since the fifth complex refractive index and the sixth complex refractive index are different, the sixth complex refractive index and the eighth complex refractive index are also different, and the sixth complex refractive index and the seventh complex refractive index are about the same in the vicinity of the second frequency, the third portion and the second portion function as optical waveguides for the incident light. Thus, the light having the first wavelength or the second wavelength which ha been inputted in the third portion heads either to the first portion or to the second portion depending on wavelength thereof, enabling the waveguide itself to function as an optical demultiplexer.
In the present invention, it has been paid attention that the response of the photonic crystal to incident light is determined by the spacial distribution of refractive index in the frequency (wavelength) of controlled light, i.e., incident light (or a certain range of frequencies when the incident light is not monochromatic light) in functioning as an optical element and is not influenced by the distribution of refractive index in other wavelength range.
Particularly, the active optical element according to the present invention uses a plurality of optical media whose refractive indices change by an external field. When the photonic crystal is formed by using such optical media, the refractive indices of two optical media out of the plurality of optical media are caused to be the same or about the same under a certain external field condition. As a result, the periodic distribution of refractive index that light senses is the distribution pattern of optical media other than the two optical media having the same refractive index.
Further, the refractive indices of two other optical media are caused to be the same under other external field condition. In this case as well, light in the optical element senses the distribution pattern of optical media other than the optical media having the same refractive index under the external field condition.
By reflecting the distribution patterns that light senses under these external field conditions on a desired crystal structure, shape of a lattice point and period, two significantly different photonic-band structures can be switched from one to the other by switching between the external field conditions.
The switchable waveguide in the photonic crystal according to the present invention, by the same principle as described above, operates by switching between the portions where the periodicity of refractive index is irregular in the crystal structure.
Further, the optical demultiplexer according to the present invention relies not on the switching of external field conditions, but on the frequency (wavelength) of incident light to cause changes in the refractive indices of optical media. In other words, the photonic crystal and the waveguide are constituted by a combination of optical media that can form a photonic crystal in which the portions that light senses as the irregularities in the periodicity of refractive index vary according to the frequency of the light, and caused to function as the optical demultiplexer.
According to the production process and the production device of the present invention, since the photonic crystal can be produced by repeating the step of forming every spot where the refractive index changes that corresponds to the same site in each unit cell at a time for the number of sites in a unit cell by shifting the optical medium as much as a minute distance by taking advantage of the translational symmetry of the crystal, they are easy to operate, and a three-dimensional photonic crystal with multiple periods in the wavelength range of light that has been virtually almost impossible to produce with the prior art can be produced in the form of a desired crystal structure with high accuracy by the production process and the production device of the present invention.
Further, according to the production process and the production device of the present invention, by changing an external field, with regard to the refractive index of the photonic crystal or the optical waveguide formed therein, a combination of optical media having the largest refractive index difference change in the frequency of controlled light to be inputted, or new spots having different refractive indices from the existing spots appear periodically, that is, a new periodic structure is formed after the change, or the ratio of the periodic peaks of the refractive indices of the media which occur in the periodic structure changes, where the response of the photonic crystal or the waveguide can be actively switched. Further, by forming spots that become a portion of the periodic structure of refractive index depending on the wavelength of the controlled light at other locations, there can be produced an optical demultiplexer in which light heads to different waveguides at a branch according to the wavelength thereof.